In Video: How a nerve agent affects the body

The UK says Novichok was used to poison ex-spy Sergei Skripal and his daughter Yulia in Salisbury.

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A police tent in Salisbury at the spot where former Russian double agent Sergei Skripal and his daughter Yulia were found critically ill (Andrew Matthews/PA)

Novichok was developed by the Soviet Union in the 1970s as a new kind of chemical weapon that would be harder to detect, more potent than existing nerve agents and exempt from the Chemical Weapons Treaty.